To move and stop automobile electronic parts such as wipers and door locks, switching circuits which turn on/off voltage supply from batteries to loads such as motors are used. Although relays are conventionally used as these switching circuits, the use of semiconductor devices is being demanded for the purposes of downsizing and power saving. Examples of loads as objects of switching control by semiconductor devices are the wiper motor and door lock motor described above, and a blower motor, a power seat motor, lamps such as a head lamp and tail lamp, a horn, a rear defogger, and a seat heater. The driving current is a few A to about 20 A, the battery rated voltage is 12 or 36 V, and the breakdown voltage is 60 to 100 V. Recently, semiconductor devices adaptable to large electric currents and high voltages of electrical automobiles such as HEV and FCV are also being required.
FIG. 21 is a circuit diagram showing an example of the conventional switching circuit using a semiconductor device. The switching circuit shown in FIG. 21 includes a charge pump circuit CP101, resistors R101 and R102, and an N-channel power MOS field effect transistor Q101 formed on a (100) plane of a silicon substrate. A power supply voltage BATT (a battery rated voltage) is 12 or 36 V. To turn on this switching circuit, a microcomputer MC outputs a high-level voltage (the battery voltage BATT). In this state, the source voltage of the transistor Q101 becomes lower than its gate voltage by the amount of a threshold voltage, so a voltage to be supplied to a load LO decreases by the threshold voltage of the transistor Q101 if the output of the microcomputer MC is directly connected to the resistors R101 and R102. Therefore, this voltage drop is avoided by raising the output of the microcomputer MC by the charge pump circuit CP 101. However, the switching circuit shown in FIG. 21 has the problems that the cost rises by the cost of the charge pump circuit CP101, and the charge pump circuit CP101 radiates noise.
FIG. 22 is a circuit diagram showing another example of the conventional switching circuit. In the arrangement shown in FIG. 21, the N-channel power MOS field effect transistor Q101 as a switching element is inserted in the high-potential side of a power supply line to the load LO. On the other hand, the switching circuit shown in FIG. 22 takes a bridge configuration in which N-channel power MOS field effect transistors Q111 and Q112 are inserted in the high-potential side of a power supply line, and N-channel power MOS field effect transistors Q113 and Q114 are inserted in the low-potential side (ground) of the power supply line. This switching circuit includes the transistors Q111, Q112, Q113, and Q114, resistors R111, R112, R113, and R114, a high-side drive circuit DR1, and a low-side drive circuit DR2. The high-side drive circuit DR1 includes a bipolar transistor and the like which drive the transistors Q111 and Q112 by amplifying the output electric current from a microcomputer MC. Likewise, the low-side drive circuit DR2 includes a bipolar transistor and the like which drive the transistors Q113 and Q114. As in the arrangement shown in FIG. 21, the switching circuit shown in FIG. 22 also requires a charge pump circuit CP 101 in order to avoid the load voltage drop, and therefore has the problems that the cost rises by the cost of the charge pump circuit CP 101, and the charge pump circuit CP 101 radiates noise.
Another method of avoiding the load voltage drop is to use a P-channel power MOS field effect transistor. Since the P-channel power MOS field effect transistor causes no such voltage drop as explained for the N-channel power MOS field effect transistor, a switching circuit can be implemented without using any charge pump circuit, so the above-mentioned problems related to the charge pump circuit can be eliminated.
Unfortunately, the current drivability, e.g., the mobility, of a P-channel MOS field effect transistor formed on a (100) plane of silicon like an N-channel MOS transistor, is about ⅓ that of the N-channel MOS field effect transistor, so the size of the P-channel MOS transistor must be made about three times as large as the N-channel MOS transistor in order to obtain current drivability equal to that of the N-channel MOS transistor by the P-channel MOS transistor. Accordingly, when a P-channel MOS transistor having characteristics equal to an N-channel MOS transistor is formed on a (100) plane of silicon, the cost is about three times that of the N-channel MOS transistor, and this poses the problem that the cost of the whole switching circuit becomes higher than those of the circuits shown in FIGS. 21 and 22 although no charge pump circuit is necessary. If the size of a P-channel MOS transistor can be made equal to that of an N-channel MOS transistor formed on a silicon (100) plane, it is possible to provide an inexpensive switching circuit in which no noise is generated from a charge pump circuit. To this end, it is necessary to make the current drivability of a P-channel MOS transistor higher than that of a transistor formed on a silicon (100) plane.
For example, patent references 1 and 2 propose the formation of a P-channel MOS transistor on a (110) plane of silicon in order to increase the current drivability of the transistor. In patent reference 1, silicon having a (100) surface on which an N-channel MOS transistor is formed is etched to form a P-channel MOS transistor on a (110) plane on a side surface. According to the findings of the present inventors, however, a P-channel MOS transistor having a gate insulation film which is a silicon oxide film formed by thermal oxidation on a (110) surface of silicon etched by the conventional method has only impractical characteristics, and cannot be used as a power transistor having a gate-to-source breakdown voltage of 10 V or more. Patent reference 2 aims to form a P-channel transistor on a (110) plane by noting the fact that, as shown in FIG. 23 (FIG. 2 of this reference), when an effective vertical electric field is about 3 V, the mobility of holes on a (110) plane is larger than that of electrons on a (100) plane. Since, however, the breakdown limit of an oxide film is 1 V as an effective vertical electric field, a P-channel MIS transistor is formed by using a high-k material such as tantalum oxide or titanium oxide as a gate insulation film without using any silicon oxide film. Even in this device, as shown in FIG. 23, the mobility is inferior to that of a normal N-channel MOS transistor, so it cannot be said that mobility equal to that of an N-channel MOS transistor is obtained.    Patent Reference 1: Japanese Patent Laid-Open No. 4-372166    Patent Reference 2: Japanese Patent Laid-Open No. 7-231088